This invention relates to the attenuation of reflected shock waves from a low pressure turbine vane component of a gas turbine engine.
Acoustic and shock waves are traveling pressure fluctuations which cause local compression of the material through which they move. Acoustic waves cause disturbances whose gradients, or rates of displacement are small—on the scale of the displacement itself. Acoustic waves travel at a speed determined by and characteristic of a given medium; thus, one must speak of the speed of sound, or acoustic speed in that medium. An acoustic wave regardless of its frequency (pitch) or amplitude (loudness) will always travel at the same speed in a given substance.
Shock waves are distinguished from acoustic waves in two key respects. First, shock waves travel faster than the speed of sound in any medium. Secondly, local displacements of atoms or molecules comprising a medium caused by shock waves are much larger than for acoustic waves. Together, these two factors produce gradients or rates of their displacement much larger than the local fluctuations themselves.
Energy is required to produce pressure waves. This is related to the equation that states that energy equals force multiplied by the displacement caused by the force. Once the driving source ceases to produce pressure disturbances, the waves decay. Attenuation involves acceleration of the natural damping process, which therefore means removing energy from pressure waves.
All matter through which pressure waves travel naturally attenuates these waves by virtue of their inherent mass. Materials possess different acoustic attenuating properties, strongly affected by density and by the presence or absence of phase boundaries and structural discontinuities. Porous solid materials, thus, are better attenuators of sound waves than perfect crystalline solids. Gases are inherently poor pressure wave attenuators.
All types of pressure waves can be reflected and diffracted by liquid and gas media. They can also be deflected or, more generally, scattered and dispersed by phase boundaries, such as liquid droplets or solid particulates suspended in air. These deflections serve to increase the distance which the wave travels. Scattering and dispersion thus produce more attenuation because they cause the transmitting pressure waves to displace more mass by virtue of the longer path. Such deflections also reduce, or may altogether eliminate the pressure waves originally traveling in a specific direction.
Acoustic Wave Attenuation
Documented efforts to reduce noise (attenuate acoustic waves) in enclosed spaces extend to the early nineteenth century. Virtually all acoustic wave attenuation concepts have been based upon layers of solid materials with significant sound absorbing properties serving as linings, coatings, or loosely-packed fibrous or granular fillers between solid layers. These sound-absorptive layers have been applied to or incorporated within structural walls, floors, ceilings, and other types of panels and partitions when acoustic attenuation is required. Several dozen patents have been granted in the United States alone which fall into this category.
In 1910, Mallock introduced the idea of using aqueous foams for noise suppression, and conducted experimental evaluation of foams in this role. See Mallock, A., “The damping of sound by frothy liquids”, Proc. Royal Soc. A84; pp. 391-5, 1910. Aqueous foams are agglomerations of bubbles, with the gas phase within each bubble completely separated from that in adjacent bubbles by aqueous liquid film comprising the bubble walls. During the years following Mallock's research, aqueous foams became widely used for fire suppression, in numerous chemical processes, and for mineral ore separation.
Not until the 1960's did interest renew in using aqueous foams for pressure wave attenuation. Research from that time and continuing to the present extended to their use for suppressing jet engine noise and acoustic disturbances arising from artillery muzzle blast, ordnance disposal, and “sonic boom” created by supersonic aircraft flight. It was during this time that researchers discovered that aqueous foams dramatically attenuate impinging shock waves.
Shock Wave Attenuation
Much more energy is required to produce shock waves compared to acoustic disturbances, which makes their attenuation more difficult. Shock waves decay to form acoustic waves when the source of the shock wave is removed or suppressed.
When traveling through gases, shock waves produce increases in pressure (often referred to as “overpressure”) and temperature; they also accelerate gas molecules and entrained particulates in the direction of shock wave travel. Shock waves produced by combustion processes, such as explosions and deflagrations, release substantial amounts of thermal and radiant energy as well. For all shock waves, the shock wave speed, overpressure, and temperature increase they induce in the local medium are mathematically linked. Attenuation of shock waves is thus achieved through directly suppressing one of these three parameters; if temperature is reduced, the overpressure and shock speed are accordingly reduced, for example.
In a high pressure turbine that is used in combination with a contra-rotating low pressure turbine, the geometry of the inlet guide vane of the low pressure turbine induces a reflection of the incident shock wave from the high pressure turbine airfoil that propagates back upstream to impact the blade. In consequence, both the performance of the high pressure turbine and its resistance to high cycle fatigue failure, i.e., its durability, is improved.
At the time of the present invention, there was no known way of accurately predicting, reducing, or eliminating this shock reflection and its attendant effects on blade vibration. The prior art reflects a solution that includes inserting a larger number of downstream low pressure turbine vanes to a level sufficient to ensure that no resonance occurs in the upstream blade row over the entire operating range of the engine. This solution necessarily leads to increased numbers of parts resulting in increased engine weight, and increased life-cycle costs over what is achieved based on the aerodynamic loading constraints of the low pressure turbine vane row of the present invention.
The use of steady jets of compressor bleed air to eliminate shock reflections in vaned contra-rotating turbines does not exist in the prior art. The present invention has distinct advantages over the current method of resonant frequency control via large airfoil count. It results in a much lighter, lower cost embodiment of the low pressure turbine vane row than was achievable in the prior art. The present invention is similar to steady film cooling, but the jet velocities are designed to cancel the incident shock from the upstream vane row instead of to provide heat-transfer management as in the case of film cooling.